1. Field of the Invention
The present invention relates to an intraluminal prosthesis for implantation into a mammalian vessel, and in particular, to an intraluminal stent that is delivered in a compressed state to a specific location inside the lumen of a mammalian vessel and then deployed to an expanded state to support the vessel. The intraluminal stent is provided with a structural configuration that maintains the prosthesis at substantially the same length in both the compressed and expanded states. The intraluminal stent may also be provided with varying rigidity or flexibility along its length.
2. Description of the Prior Art
Intraluminal prosthesis, such as stents, are commonly used in the repair of aneurysms, as liners for vessels, or to provide mechanical support to prevent the collapse of stenosed or occluded vessels. These stents are typically delivered in a compressed state to a specific location inside the lumen of a vessel or other tubular structures, and then deployed at that location of the lumen to an expanded state. The stent has a diameter in its expanded state which is several times larger than the diameter of the stent in its compressed state. These stents are also frequently deployed in the treatment of atherosclerotic stenosis in blood vessels, especially after percutaneous transluminal coronary angioplasty (PTCA) procedures, to improve the results of the procedure and to reduce the likelihood of restenosis.
The positioning of a stent at the desired location in the lumen of a body vessel is a critical factor that affects the performance of the stent and the success of the medical procedure. Since the region in a lumen at which the stent is to be deployed is usually very difficult for a physician to access, it is essential that the stent""s deployed diameter and length be known before the physician can accurately position a stent with the correct size at the precise location. For example, since the diameter and the length of the diseased or damaged segment or region of the body vessel can vary for different body vessels, disease states, and deployment purposes, it is important that a stent having the precise diameter and length be delivered to this region for deployment.
Careful sizing of this region of the lumen of the body vessel may pose a difficult challenge for many physicians who know the exact dimensions of the body vessel at this region, but are not certain about the stent""s deployed diameter and length. This is due to a foreshortening effect which is experienced by many stents when they are expanded from their compressed state to their expanded state.
This foreshortening effect is illustrated in FIGS. 1A, 1B, 2A and 2B, which illustrate portions 20 of a stent having a mesh-like pattern made up of V-shaped struts or legs 22 and 24 connected at their apices 26. Two pairs of these V-shaped struts 22, 24 are illustrated in this portion 20 of the stent. Each of these struts 22 and 24 has a length h. FIG. 1B illustrates the portion 20 of the stent in a fully compressed state, in which the length h has a longitudinal or horizontal component l2 (see FIG. 2B), and FIG. 1A illustrates the same portion 20 of the stent in a fully expanded state, in which the length h has a longitudinal or horizontal component l1 (see FIG. 2A). As illustrated by the imaginary lines 28 and 30 in FIGS. 1A and 1B, and in FIGS. 2A and 2B, l1 is shorter than l2 because the angle which the strut 22 assumes with respect to the horizontal axis is greater when in the expanded state, so the length of the expanded portion 20 is shorter than the length of the compressed portion 20 by a length of 2d. This foreshortening is caused by the shortening of the longitudinal component 1 of the struts 22 and 24 as the stent is expanded from the compressed state to the expanded state.
This foreshortening effect is troublesome because it is not easy to determine the exact dimension of this foreshortened length 2d. The physician must make this calculation based on the material of the stent, the body vessel being treated, and the expected diameter of the stent when properly deployed in the lumen of the body vessel. For example, the foreshortened length 2d will vary when the same stent is deployed in vessels having different diameters at the region of deployment.
In addition, there are certain body vessels that experience a change in vessel lumen diameter, anatomy or disease state along their lengths. Stents to be deployed at such vessels will need to be capable of addressing or adapting to these changes.
An example of such a body vessel are the carotid arteries. Blood is delivered from the heart to the head via the common carotid arteries. These arteries are approximately 8-10 mm in lumen diameter as they make their way along the neck up to a position just below and behind the ear. At this point, the common carotid artery branches into a 6-8 mm lumen diameter internal carotid artery, which feeds blood to the brain, and a 6-8 mm lumen diameter external carotid artery, which supplies blood to the face and scalp. Atherosclerotic lesions of the carotid artery tend to occur around this bifurcation of the common carotid artery into the internal and external carotid arteries, so stents often-need to be deployed at this bifurcation.
Another example are the iliac arteries, which have a lumen diameter of about 8-10 mm at the common iliac artery but which decrease to a lumen diameter of about 6-7 mm at the external iliac artery. The common iliac arteries experience more localized stenosis or occlusive lesion which are quite often calcific and usually require a shorter stent with greater radial strength or rigidity. More diffused atherosclerotic disease of the iliac system will commonly involve both the common and external iliac arteries, and necessitate a longer stent having increased flexibility that is suitable for deployment in the tortuous angulation experienced by the iliac system.
The femoropopliteal system similarly experiences localized and diffused stenotic lesions. In addition, the flexibility of a stent is important where deployed at locations of vessels that are. affected by movements of joints, such as the hip joint or the knee joint.
The renal arteries provide yet another useful example. The initial 1 cm or so at the orifice of a renal artery is often quite firmly narrowed due to atheroma and calcification, and is relatively straight, while the remainder of the length of the renal artery is relatively curved. As a result, a stent intended for implantation at the renal arteries should be relatively rigid for its first 1.5 cm or so, and then become more flexible and compliant.
Thus, there remains a need for an intraluminal prosthesis that maintains a consistent length in both its fully compressed and fully expanded states, and in all states between its fully compressed and fully expanded states. There also remains a need for a stent which can accomodate body vessels having varying lumen diameters, different anatomies, and different disease states.
In order to accomplish the objects of the present invention, there is provided a stent having a plurality of annular elements. Each annular element has a compressed state and an expanded state, and has a longitudinal dimension which is smaller in the expanded state than in the compressed state. A plurality of connecting members connect adjacent annular elements, with the connecting members operating to compensate for the smaller longitudinal dimension of each annular element in the expanded state.
In one embodiment of the present invention, each annular element includes a plurality of struts and apices connected to form an annular configuration. The connecting members are connected to the apices of the adjacent annular elements. The plurality of struts of the annular elements include left and right struts, with each pair of left and right struts connected to each other at an apex. Each strut has a longitudinal dimension which is smaller when the annular element is in the expanded state than in the compressed state.
In one embodiment of the present invention, at least one of the annular elements may have a closed configuration such that the plurality of alternating struts and apices are connected to each other to form a closed annular element. In addition, it is also possible for at least one of the annular elements to assume an open configuration such that the plurality of alternating struts and apices are not connected at at least one location.
In a preferred embodiment of the present invention, the connecting members have a plurality of alternating segments. In one embodiment, the connecting members have a plurality of alternating curved segments defining alternating top and bottom curved apices. In another embodiment, the connecting members have a plurality of alternating curved and straight segments. In a further embodiment, the connecting members have a plurality of alternating and angled straight segments. The connecting members have a larger longitudinal dimension when each annular element is in the expanded state than in the compressed state to compensate for the smaller longitudinal dimension of the annular element in the expanded state.
The stent according to the present invention further includes a plurality of apertures defined by adjacent annular elements and connecting members. In one embodiment, it is possible for the apertures of different segments of the stent to have different sizes.
The stent according to the present invention further provides a plurality of segments, at least two of which have a different degree of flexibility. In one embodiment, the varying flexibility is accomplished by forming a plurality of gaps. These gaps may be formed by omitting one or more of the connecting members, or portions of connecting members, between adjacent annular elements, or by omitting one or more of the struts, or by omitting connecting members and struts. In another embodiment, the varying flexibility is accomplished by providing the apertures of different stent segments with different sizes.
The stent according to the present invention may further provide segments that assume different diameters when the stent is in its expanded state. The differing diameters may be accomplished by providing the stent in a tapered or a stepped configuration.
In a preferred embodiment according to the present invention, the stent is made from a shape memory alloy, such as Nitinol, although other materials such as stainless steel, tantalum, titanium, elgiloy, gold, platinum, or any other metal or alloy, or polymers or composites, having sufficient biocompatibility, rigidity, flexibility, radial strength, radiopacity and antithrombogenicity can be used for the stent material.
Thus, the stent according to the present invention maintains a consistent length in both its fully compressed and fully expanded states, and in all states between its fully compressed and fully expanded states. As a result, the stent according to the present invention facilitates accurate sizing and deployment, thereby simplifying, and possibly reducing the time needed for, the medical procedure. In addition, the stent according to the present invention provides varying flexibility and rigidity along its length and/or circumference, as well as varying diameters along different segments of the stent, thereby facilitating the treatment of body vessels having varying lumen diameters, different anatomies and different disease states.